FOCUS ON: Oscillatory Rheometry & Quality Control

The controlling factor in all polymer conversion processes is the material's rheological behaviour - if you can't deform & make the material flow, you can't process it! The rheological response of a polymer melt is also highly sensitive to material structure. A rotational rheometer in oscillation mode (Oscillatory Rheometry), can detect discrete changes in structural properties, therefore affording a precise & accurate method of providing quality control for all aspects of the process. Whether it be screening incoming raw materials to discriminate batch to batch variance, optimising the conversion process through minimising the effects of thermal & shear history, or determining the cause of degradation in failed components, oscillatory rheometry can provide this information in a fast & reproducible manner.

As mentioned, Oscillatory rheometry measurements are made utilising a rotational rheometer. This type of rheometer is an extremely sensitive, yet robust bench top instrument which is available in a number of machine configurations. The measurement system of the instrument is modular and therefore a wide range of test parameters can be derived in conjunction with the use of either plate / plate, cone & plate or coaxial cylinder systems.

Some examples of the type data that can be produced on the instrument and their applications are demonstrated below:

Simple 'Go / No Go' quality control programmes

	Controlling quality of incoming batches of material is critical to maintaining an optimised conversion process and minimising reject rates. The graph on the left demonstrates a simple 'Go / No Go' quality control programme set up for an automotive components moulder who operated a 'just in time' manufacturing regime. The company had been experiencing problems with variation in the properties of supplied batches of Polypropylene, evident in fluctuating levels of product reject rates - which threatened planned delivery schedules. By characterising samples taken from 'Good' & 'Bad' batches of material a simple 'process window' (the limits of which are indicated by the green curves on the graph), was compiled against which future batches could be compared - enabling any incoming materials that fell outside of the window to be rejected before entering the production line. Application of Rheology to Failure Analysis
Another major application of oscillatory rheometry is the support of failure analysis projects. The slide on the right displays data produced as part of one such project, carried out for an electrical connectors manufacturer. Problems arose with a particular product which had a brass fitting pressed into a central boss within the moulding. This product was sourced from two different manufacturing facilities - one in Bulgaria which produced mouldings with no problems and one in Poland which was producing components that cracked upon pressing the brass insert into position. The connector was moulded out of a glass-filled PolyButylene Terephthalate (PBT) grade, a semi-crystalline, hygroscopic (moisture sensitive) material. Sections removed from both 'good' and failed components were rheologically characterised and it could be seen there was almost a full (log) decade drop in complex viscosity across the measured frequency range between the samples. This considerable decrease indicated there had been a sharp reduction in molecular weight due to degradation - chain scission probably being a result of inadequate pre-process drying.
To confirm this theory, test specimens were injection moulded after zero, one, two & three hours drying in a vacuum oven @ a temperature of 135°C. Sections of the dried mouldings and virgin granules were measured and it could be seen that the viscosity response of the undried (zero hours) moulding lays closest to the failed sample's viscosity curve. As predicted, as drying time was increased the level of degradation decreased demonstrated by an increased retention of properties. At the optimised drying time for PBT of 3 hours @ 135°C it could be seen the viscosity had risen to a level comparable to that of the 'good' sample although interestingly, after only one hours drying, properties increased to a similar level.
Cure Kinetics of Thermosetting Materials	Due to the modular nature of the rotational rheometer measurement system, disposable plate / plate systems can be used to perform tests on curing and/or corrosive materials. Measurements are usually carried out either isothermally at fixed strain & frequency as a function time (Isothermal cure), or fixed strain & frequency as a function of temperature, (reactive viscosity measurements). The plot to the left depicts the viscosity responses of an NBR rubber compound as a function of different heating rates, at 3 different frequencies. Running as a function of temperature in cooling mode is also excellent at determining phase transitions in hot melt adhesives and investigating the effect of changing ingredient ratios in compound trials.
Control software for modern rotational rheometers are exceptionally versatile. A useful function of software is the ability to 'string' individual test programmes into a project 'macro'. The graph on the right is an example of this. A manufacturer of products for the offshore oil industry was encapsulating components in a large volume of Polyurethane, for use in offshore applications. The cured resin was required to reach a specific modulus to ensure integrity of the moulding at the pressures experienced in deep sea conditions. Full cure of the moulding was not reached until almost 24 hours after mixing and casting, and the cure kinetics passed through three distinct phases: an initial exothermic reaction which raised the temperature of the casting from 20°C to 130°C, an hour at a constant 130°C (a result of adiabatic heating), followed by an 18 hour slow cooling phase. This was easily replicated by using three test templates joined together in a project macro. All templates used the same fixed frequency & strain amplitude with the first programme running a heating ramp from 20°C to 130°C @ a heating rate of 20°C/min (to replicate the heat generated during the initial exothermic reaction). The second programme was an isothermal measurement @ 130°C for one hour, (simulating adiabatic heating conditions), and the third programme an extremely slow cooling ramp from 130°C to 20°C over 18 hours (-0.102°C/min).